The present disclosure relates generally to vehicle steering control systems and, more particularly, to a mechanically linked active steering system.
Conventional vehicular steering systems have an articulated mechanical linkage connecting an input device (e.g., steering wheel or hand-wheel) to a steering actuator (e.g., steerable road wheel). Even with power assisted steering in an automobile, for example, a typical hand-wheel motion directly corresponds to a resulting motion of the steerable road wheels, substantially unaffected by any assist torque.
However, for a vehicular steering system with active steering, such as that used in an automotive front-controlled steering system, a given motion of the hand-wheel may be supplemented by an additional motion, such as that from a differential steering actuator, which translates into a motion of the steerable road wheels that does not necessarily correspond to the given motion of the hand-wheel. Consequently, when the differential steering actuator is inactive, the motion of the steerable road wheels directly corresponds to the hand-wheel motion due to the articulated mechanical linkage, just as in conventional systems.
The term “active steering” relates to a vehicular control system, which generates an output that is added to or subtracted from the front steering angle, wherein the output is typically responsive to the yaw and/or lateral acceleration of the vehicle. It is known that, in some situations, an active steering control system may react more quickly and accurately than an average driver to correct transient handling instabilities. In addition, active steering can also provide for continuously variable steering ratios in order to reduce driver fatigue while improving the feel and responsiveness of the vehicle. For example, at very low speeds, such as that which might be experienced in a parking situation, a relatively small rotation of the hand-wheel may be supplemented using an active steering system in order to provide an increased steering angle to the steerable road wheels.
Prior devices act to modify the relationship between driver input and steering output by providing a supplemental power source within the steering system that actively augments the position of the wheels or acts to augment the control of the primary steering power source. Examples include (1) the addition of a second axially actuated device in addition to the primary axial translating device (e.g., hydraulic assisted steering rack), and (2) addition of a motor driven differential device between the operator and the steering valve of a typical hydraulic power steering system. In each case, additional power is added to the system through the added component to affect steering augmentation and in each case a portion of that power is transmitted to the operator as secondary feedback. Also noteworthy is the requirement in each case that the driver provide the upstream reaction to the system input in order for the desired steering change to be realized. Additionally, in the example number two, any lash in the differential will be directly felt by the operator.
Without operator reaction, most of the system input will be directed to the operator input device (i.e., steering wheel) and result in no change to the vehicle path. Conversely, steer-by-wire systems have the ability to directly control the primary steering actuator to affect the operator-to-steerable device kinematic relationship. However, steer-by-wire systems do not maintain a full-time mechanical link between the operator and the steerable device.
Thus, it is desirable to provide active steering orientation of the steerable device directly, as in by-wire systems, and maintain a mechanical link between the operator input and steerable device, as in prior active steer systems, while isolating the operator to some degree from such steerable device orientation modifications and associated feeback.